1. Field of the Invention
The present invention relates to an air-fuel ratio control device of an internal combustion engine that performs feedback control of a fuel quantity injected into the internal combustion engine based on a sensing value of an air-fuel ratio sensor sensing an air-fuel ratio of exhaust gas of the internal combustion engine.
2. Description of Related Art
A recent electronically-controlled automobile has an air-fuel ratio sensor arranged in an exhaust pipe for sensing an air-fuel ratio or an oxygen concentration of exhaust gas of an internal combustion engine. Feedback control of a fuel quantity (or an air-fuel ratio of a mixture gas) injected into the internal combustion engine is performed to maintain the air-Fuel ratio of the exhaust gas to the vicinity of a target air-fuel ratio based on the output of the air-fuel ratio sensor. Thus, exhaust emission and fuel consumption are improved. In such the air-fuel ratio feedback control system, if a response characteristic of the air-fuel ratio sensor sensing the air-fuel ratio of the exhaust gas is degraded, sensing accuracy of the air-fuel ratio is deteriorated and air-fuel ratio control accuracy is deteriorated, leading to deterioration of the exhaust emission and the like.
As measures against such the problem, there is a system (for example, as described in Patent document 1: Japanese patent No. 3581737) that determines presence/absence of degradation of a response characteristic of an air-fuel ratio sensor based on an adaptation parameter used for feedback control and decreases a gain of the feedback control if the degradation of the response characteristic of the air-fuel ratio sensor is detected through the determination.
In general, the air-fuel ratio feedback control system is designed by modeling a dynamic characteristic of a control object since a fuel supply quantity to an engine is changed until an output of an air-fuel ratio sensor changes using a dead time plus a first-order lag characteristic (a response time). In order to meet the exhaust gas regulations (i.e., demands for low emission), which will become more and more severe in the future, it has been increasingly required to reduce the deterioration of the air-fuel ratio control accuracy due to the degradation of the dead time or the degradation of the response time.
The degradation of the dead time and the degradation of the response time occur respectively and individually, and the degradation of one of them can advance ahead of the other. Therefore, it is difficult to perform the feedback control corresponding to the degradation of the dead time or the degradation of the response time only by decreasing the gain of the feedback control when the degradation of the response characteristic of the air-fuel ratio sensor is detected as described in above Patent document 1.
Recently, it has been found out that the degradation of the response characteristic of the air-fuel ratio sensor occurs asymmetrically between a rich side and a lean side of the air-fuel ratio. Therefore, a certain technology (e.g., refer to Patent document 2: JP-A-2007-187129) senses a dead time and a time constant (i.e., a response time) of an air-fuel ratio sensor on both of the rich side and the lean side respectively and separately. An average value of the sensing values on the rich side and an average value of the sensing values on the lean side are calculated respectively and compared with reference values respectively, thereby detecting degradation of the air-fuel ratio sensor.
The technology described in Patent document 2 only performs the degradation diagnosis of the air-fuel ratio sensor by detecting the asymmetrical degradation of the air-fuel ratio sensor between the rich side and the lean side but does not have any function to reflect the detection result of the asymmetrical degradation in the feedback control. Therefore, the technology cannot reduce the deterioration of the air-fuel ratio control accuracy due to the asymmetrical degradation.